Light emitting devices, such as incandescent devices, florescent devices, high intensity discharge, and light-emitting diode(s) (LED) devices all have different visual light emission spectrum or spectra—the spectrum of frequencies of electromagnetic radiation in the visible range emitted from the particular light emitting device—as shown in FIG. 1. For example, while LED light devices have the distinct economic advantages of using less energy and lasting many years longer than a traditional incandescent light device, the aesthetic quality of the light emitted from an LED device is less pleasing to the eye than the traditional incandescent device. Traditional incandescent devices, on the other hand, emit a more full color spectrum that, when viewed by consumers, is perceived as an aesthetically pleasing as it is closer to “white” light. White light is simply the color the human eye sees when it looks at light which substantially contains all the wavelengths of the visible spectrum. Such white light stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts. White light has become the standard, expected, or gold standard of light for humans. For example, light emitting devices are typically graded in their ability to reproduce the colors of an object in comparison with a natural, or “perfect,” light source that produces white light, like sunlight. When colors are viewed from a light source that includes an emission spectrum with missing or low irradiance of one or more wavelength (i.e., color), some colors may appear “unnatural.” Because of the importance of white light, and since white light is the mixing of multiple wavelengths of light in the visible spectrum, there have arisen multiple techniques for characterization of light that relate to how humans interpret a particular light emitted form a light emitting device.
One technique for the characterization or grading of the spectrum of light emitted from light emitting devices is the color temperature, which provides an index to the “whiteness” of the white light. Correlated color temperature (CCT) is characterized in color reproduction fields according to the temperature in degrees Kelvin (K) of a black body radiator that radiates the same color light as the light in question. A cooler white light, similar to the light generated by commercial fluorescent lamps, has a higher CCT, whereas a warmer white light, similar to the light generated by residential incandescent lamps, has a lower CCT. Direct sunlight has a color temperature of about 4,874 K, while indirect sunlight has a white color temperature of about 6,774 K, and an incandescent device has a white color temperature of about 2854 K. A color image viewed at 3,000 K will have a relatively reddish tone, whereas the same color image viewed at 10,000 K will have a relatively bluish tone. All of this light may be referred to as “white,” but it has varying spectral content.
A second term applied to identify the color of the light source regardless of its lighting level or lumen is the chromaticity of the light source. When the chromaticity of different light sources is equal, the color of the light from each light source appears the same to the eye regardless of the lighting level. Chromaticity is measured in coordinates based upon a standard developed by the Commission Internationale de l'Eclairage (CIE) in 1931. An example of such coordinates is shown in Table 1, which identifies the X, Y chromaticity coordinates for the listed white light sources. FIG. 1B graphically illustrates the color and wavelength of the emitted light represented by X,Y coordinates based upon the CIE 1931 Standard, and highlights the rather large coordinate space covered by what is generally called “white” light.
TABLE 1White Points Defined in CIE 1931 StandardColorNameXYTemperatureCommentsA0.44760.40752854° KIncandescent LightB0.38400.35164874° KDirect SunlightC0.31010.31626774° KIndirect SunlightD50000.34570.35865000° KBright IncandescentLightD65000.31270.32976504° K“Natural” DaylightE0.33330.33335500° KNormalized Reference
Another classification of white light involves its quality. In 1965 the CIE recommended a method for measuring the color rendering properties of light sources based on a test color sample method. In essence, this method involves the spectroradiometric measurement of the light source under test. This data is multiplied by the reflectance spectrums of eight color samples. The resulting spectrums are converted to tristimulus values based on the CIE 1931 standard observer. The shift of these values with respect to a reference light are determined for the uniform color space (UCS) recommended in 1960 by the CIE. The average of the eight color shifts is calculated to generate the general color rendering index, known as CRI.
The color rendering index (CRI), sometimes called color rendition index, is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source, like sunlight—white light. In general terms, CRI is a measure of a light source's ability to show object colors “realistically” or “naturally” compared to a familiar reference source, either incandescent light or daylight. Numerically, the highest possible CRI is 100, for a Black body, dropping to negative values for some light sources. Because sunlight possesses every wavelength of visible light, it has a CRI of 100. A lower CRI means that the light source has a lower number of wavelengths of visible light. Low-pressure sodium lighting has negative CRI; fluorescent lights range from about 50 for the basic types, up to about 90 for the best tri-phosphor type. Typical LEDs have about an 80 CRI value.
CRI is calculated from the differences in the chromaticities of eight CIE standard color samples (CIE 1995) when illuminated by a light source and by a reference illuminant of the same correlated color temperature (CCT); the smaller the average difference in chromaticities, the higher the CRI. A CRI of 100 represents the maximum value. Lower CRI values indicate that some colors may appear unnatural when illuminated by the lamp. Within these calculations the CRI is scaled so that a perfect score equals 100, where perfect would be using a source spectrally equal to the reference source (often sunlight or full spectrum white light). For example a tungsten-halogen source compared to full spectrum white light might have a CRI of 99 while a warm white fluorescent lamp would have a CRI of 50. Of note, although prior art device techniques may achieve nearly “white” light, the missing red light wavelengths therein do not produce a light that is acceptable in all applications or for all consumers. For example, in some applications or for some consumers, such as those who value or rely on aesthetics and, in particular, color, the nearly “white light” emitted by prior art devices is simply insufficient or unacceptable.
Another technique for the characterization or grading of the spectrum of light emitted from light emitting devices is the color quality scale (CQS). CQS, which was developed by researchers at NIST Development of a Color Quality Scale, evaluates several aspects of the quality of the color of objects illuminated by a light emitting device. CQS involves several facets of color quality, including color rendering, chromatic discrimination, and observer preferences. More specifically, CQS is a quantitative measure of the ability of a light source to reproduce colors of illuminated objects. The output of this Color Quality Scale (CQS) is a composite score incorporating a light emitting device's ability to accurately render object colors, permit precise discrimination between different colors, and display object colors in a way that is visually pleasing to typical users (i.e., humans).
The method for calculating the CQS is derived from modifications to the method used in the CIE's color rendering index (CRI) discussed above. As discussed above, CRI is based on only eight reflective samples. The eight reflective samples are all relatively low to medium chromatic saturation. Some experts argue the eight reflective samples colors of the CRI do not adequately span the range of normal object colors. For example, some light emitting devices that are able to accurately render colors of low saturation perform poorly with highly saturated colors, particularly the peaked spectra of LEDs. Instead of the eight reflective samples of the CRI, the CQS utilizes fifteen Munsell samples to calculate the CQS. Further, the CQS factors in the root-mean-square (RMS) of all color shifts, rather than just averaging all of the samples like the CRI. The CQS also differs from the CRI in that the CRI penalizes light emitting devices for showing increases in object chromatic saturation compared to reference lights, which is actually desirable for most applications. To incorporate observer preference, the CQS differentiates between hue and saturation shifts and takes their directions into account.
The scales or measurements of the emission spectrum (i.e., color) of light emitting devices described above (and others) are needed because current light emitting devices fail at producing white light. With regards to LEDs, single or unitary “white light” LED chips currently do not exist. The most commonly used LED chips emit a cold blue light. The visual colors missing from the basic blue LED light spectrum are the red wavelengths. The diagram shown in FIG. 1 shows the spectrum of visible light emitted by natural daylight (sunlight) 11, LED light devices 13, traditional incandescent light devices 15, and compact florescent light devices 17. As shown by the arrow “R”, LED light devices fail to produce red or deep red color light—wavelengths greater than about 600 nanometers to about 650 nanometers. Prior attempts to achieve an LED light device or lamp that produces white light, or nearly white light 1 (i.e., that warm the “cool” light created by LEDs), fall generally into two broad categories as illustrated in FIG. 3: (A) changing the color of the light emitted by the LED 19 through either the use of phosphor 21 or another luminescent or incandescent composition within the device (e.g., within the hood) (shown as “A” in FIG. 3); use of multiple color-mixed LEDs 19′ within the device (shown as “B” in FIG. 3); and (3) changing the color of multiple color-mixed LEDs by doping the device envelope 25 (e.g., the hood) with phosphor or another luminescent or incandescent composition, or outside of the device hood via an outer substrate phosphor or another luminescent or incandescent composition.
Blended luminescent, incandescent or phosphor mechanisms (e.g., blended phosphor or other luminescent or incandescent coatings) are currently the dominant method of warming the naturally cool light of an LED chip to more closely mimic the white light emitted by an incandescent device or white light. As shown as “A” in FIG. 3, the single spectrum LED chip 19, which typically emits a dominant blue light (i.e., dominate blue wavelength), is covered or set behind a phosphor coating or member (or another luminescent or incandescent composition) 21 which substantially absorbs light emitted from the LED chip or die 19 and emits a warmer, broader light spectrum (e.g., “yellow” light/wavelengths). While the goal of the addition of phosphor to an LED chip is to produce white light, the phosphor typical adds only yellow tones and the LED is still missing, or has only a relatively small amount of, the red light array or wavelengths of the white light spectrum. Stated differently, the use of phosphor warms the light, but produces a slightly greenish-yellow colored light, and not truly white light. To the human eye, the resulting “white” light emitted by LED devices with luminescent, incandescent or phosphor mechanisms 21 is often describes as very sterile, harsh, and unnaturally “cold” light. However, the relative low cost of this technology and the general adequacy of the light color and quality make LED devices with luminescent or incandescent or phosphor mechanisms the most commonly used light-conditioning “solution” of LED light devices or lamps.
Current LED devices with luminescent or incandescent or phosphor mechanisms (e.g., a chemical phosphor or powdered mineral slurry applied to the LED device) include further drawbacks. For example, a drawback of current LED devices with luminescent or incandescent or phosphor mechanisms involves the delicate nature of the phosphor material. Phosphors are adversely affected by both heat, which the LEDs generate, and humidity. A phosphor disc, in which the phosphor is contained inside an enclosed plastic disc, is occasionally used as an attempt to help to alleviate these drawbacks. Yet another drawback of current LED devices with luminescent or incandescent or phosphor mechanisms is that they tend to not completely produce white light, as explained above. In essence, while the phosphor may provide the missing yellow spectrum of the light emitted by the LED diode chip, the phosphor fails to introduce red spectrum(s). Still another drawback of current LED devices with luminescent or incandescent or phosphor mechanisms involves the manufacturing process of such devices or lamps. Due to the extreme sensitivity of the chemicals and ratios used in the manufacture of a suitable blue LED base chip and phosphor, the smallest variance in these elements or the conditions under which they are applied, results in wildly varying shades of blue-white. This problem is so prevalent that production runs of “white” LEDs with phosphor must be sorted into multiple categories in order to provide the user with devices having relatively consistent optical outputs. However, even within a category, the tolerances are broadly applied because no two LED devices with luminescent or incandescent or phosphor mechanisms emit consistent light spectrum output.
As shown as “B” in FIG. 2, another alternative method of warming the naturally cool light of an LED light device to more closely mimic the white light emitted by an incandescent device is to utilize multiple LED chips or diodes of different colors 19′. For example, some devices or lamps include three or more separate LED chips of different colors 19′ into one tri-chromatic or multi-chromatic device and “tune” them via mixing optics 23 to attempt to replicate white light. A blue LED chip, a green chip, a red chip, an amber chip, and so on are all packaged together to create a re-combined full spectrum of white light. Each individual chip output must be adjusted via mixing optics 23 in order to blend the various light colors into a white spectrum light. The most successful attempts with this process have used six or more different LED chips 19′, each emitting a different wavelength of light, in order to more closely approximate natural light. The mechanics of this approach are often described with the analogy to recombining the light bands that are created when light passes through a prism.
Current LED devices with multiple LED chips or diodes 19′ (e.g., embedding three or more different LED chips—one blue, one red, and one green—in a single device and “tuning” via mixing optics 23) to approximate white light include several drawbacks. Foremost, the inclusion of multiple LED chips 19′ within a single device or lamp is relatively expensive, and often cost prohibitive. Another drawback of multiple chip or diode 19′ LED devices is that they tend to produce a relatively large amount of heat. The heat output from three or more LED chips 19′ is relatively greater than the heat output of one chip, thereby requiring different engineering of the device to adequately dissipate the heat to protect the chips. Yet another drawback of multiple chip or diode 19′ LED devices is that tuning the multiple chips 19′ via mixing optics 23 is relatively difficult and time consuming. Each individual chip must be tuned via mixing optics 23 and balanced to attempt to recreate the metameric effect of incandescent light.
As shown as “C” in FIG. 2, yet another alternative method of warming the naturally cool light of an LED device to more closely mimic the white light, or an incandescent spectrum, is to utilize a hybrid or combination of multiple LED color-mixed chips or diodes 19′ with mixing optics 23 and luminescent or incandescent or phosphor 21. The hybrid approach attempts to draw strengths from each of the two methods “A” and “B” described above by using a combination of various discrete colored LEDs 19′ via mixing optics 23 and luminescent or incandescent or phosphor 21. While this approach does a relatively good job of producing white light similar to incandescent devices, the approach includes several drawbacks that mirror the problems of each invention standing alone (as described above).
Unfortunately, the drawbacks of typical LED emission spectrums and tuning “solutions” thereof is not exclusive to LED light emitting devices. For example, as shown in FIG. 1, typical compact florescent light devices 17 produce an emission spectrum that includes a high irradiance of several narrow ranges of violets (e.g, wavelengths between about 375 nanometers and 400 nanometers) greens (e.g, wavelengths between about 500 nanometers and 550 nanometers) and oranges (e.g, wavelengths between about 600 nanometers and 625 nanometers). However, all other wavelengths in the visible spectrum of compact florescent light devices 17 may include relatively substantially low irradiance. Similarly, as also shown in FIG. 1, while incandescent includes a high irradiance of the “warm” colors (i.e., relatively high wavelengths, such as over 600 nanometers), they include low irradiance of “cool” colors (i.e., relatively low wavelengths). As such, all current light emitting devices produce emission spectrum that differs from white light (e.g., sunlight) that includes high irradiance of all of the wavelengths (i.e., colors) of the visible spectrum. As a result, current light emitting devices may benefit from tuning methods and resulting apparatus that alter the emission spectrum of the devices, such as modifying the emission spectrum (or at least a portion thereof) closer to that of white light. For example, tuning methods and resulting apparatus may increase the CRI and or CQS of a light emitting device. Further, because of varying user preferences or needs (e.g., aesthetic considerations), current light emitting devices may benefit from tuning methods and apparatus that can alter the emission spectrum to suit user preferences or needs. For example, emission spectrum tuning methods and resulting apparatus may advantageously tune one light emitting device such that the resulting emission spectrum is similar, or a least closer, to that of another light emitting device. As another example, emission spectrum tuning methods and resulting apparatus may advantageously tune a light emitting device to suit the particular “color” requirements of a particular use or desires of a particular user.
Accordingly, it is an object of the present invention to overcome one or more of the above-described drawbacks and/or disadvantages of the prior art while achieving these needs.